Digital halftoning with clustered microdots

ABSTRACT

A method of reproducing a continuous-tone image with a printing press comprises the step of making a printing plate having a halftone raster image comprising regularly tiled halftone cells which consist of a grid of image pixels and non-image pixels; the halftone cells are obtained by digital halftoning with a single threshold array and at least a portion of the halftone cells in the raster image comprise multiple image clusters, image clusters being defined as mutually separated groups of more than 4 adjacent image pixels; the image clusters allow to obtain a printed image of the same image density with less ink than with conventional screens wherein the image pixels are grouped into a single cluster.

TECHNICAL FIELD

The invention relates to the field of digital halftoning methods usedfor printing images, in particular by means of lithographic orflexographic printing presses.

BACKGROUND ART

Printing presses and digital printers cannot vary the amount of ink ortoner that is applied to particular image areas except through digitalhalftoning, also called dithering or screening. Binary digitalhalftoning is the process of rendering the illusion of a continuous-toneimage by means of a halftone raster image (also called “screen”) whichconsists of pixels (also called “microdots”), which are either “on”(image pixels) or “off” (non-image pixels), i.e. which correspondrespectively to image areas and background areas of the image.

These pixels are typically arranged as a grid in so-called halftonecells. The ratio of image pixels versus non-image pixels in a halftonecell defines the relative image density, also dot percentage, of thehalftone cell. A halftone cell wherein half of the pixels are imagepixels and half are non-image pixels, has a relative image density of50%. Halftone cells which represent highlights (white areas) of theimage, have an image density which is lower than 50% and halftone cellswhich represent shadows (darker areas) of the image, have an imagedensity which is higher than 50%.

Digital halftoning is a well-known technique, which is explained in e.g.“Digital Halftoning”, MIT Press, 1987, ISBN 0-262-21009-6, whereinchapter 5 about ‘clustered-dot ordered dither’ is background art for thepresent invention including the use of threshold arrays for renderingcontinuous-tone images. Another overview of digital halftoning methodsis disclosed in the article “Recent trends in digital halftoning”, Proc.SPIE 2949, Imaging Sciences and Display Technologies, (7 Feb. 1997);doi: 10.1117/12.266335 wherein also multilevel digital halftoning isexplained.

The most commonly used screening technique produces so calledamplitude-modulated (AM) screens. AM screens consist of halftone cellswherein the image pixels are grouped in a single cluster, which is oftencalled a “(halftone) dot” or “AM dot”, not to be confused with the abovementioned microdot (=pixel). In AM screening, a higher relative imagedensity is obtained by growing the size of the dot cluster. Whenprinting an AM image, e.g. with a printing press or a digital inkjetprinter, each AM dot corresponds to a certain amount of ink, calledfurther a blob, which is pressed or jetted onto the substrate to beprinted, dried and/or cured. Especially when multiple inks (colourselections) are to be printed on top of each other, whether wet-on-wetor wet-on-(semi)dry, the spreading of the ink, which is determined bythe thickness of the blob and the local (de)wetting on and/or theabsorption of the ink by the substrate, renders the printed blob locallyuncontrollable. As a result, the printed image may show noise, and thisproblem is often depended on the nature of the substrate.

Such problems can be addressed by other screening technologies such asFM (frequency modulated) screening or techniques involving errordiffusion. In both these techniques, the microdots are not grouped intoclusters but spread more or less randomly over the image. The localimage density is modulated by the frequency of the microdots. Howeveralso these techniques are characterized by other issues like printstability, poor smoothness of flat tones, higher dot gain and higherwear of printing plates in long print runs.

Hybrid screening techniques are available which combine AM and FMmethods so as to obtain the advantages of both. Said screeningtechniques however involve the use of multiple threshold arrays forrendering a continuous-tone image, which requires more memory space tostore these multiple threshold arrays, for example a threshold arraywith FM method in the highlights, a threshold array with AM method inthe midtones and another threshold array with FM method in the shadows.In addition, the transitions from one threshold array to another mayproduce density jumps in the printed image whereby calibration of saidscreening techniques also takes more service time than AM and FM.

US2007/0002384 discloses a method of controlling thickness of an inkblob in an AM halftone region of a printing plate or an intermediateimage carrier on a digital press. The method generates a raster image ofregularly tiled halftone cells wherein the image pixels are arranged toform concentric rings, which enclose an area of non-image pixels. As aresult, the extent at which the ink can spread within the cell remainslimited because no further spreading of the ink is possible as soon asthe enclosed area is filled.

The latter problem is solved in copending application PCT/EP2018/079011,which was filed on 23 Oct. 2018, and which discloses a halftone rasterimage comprising a plurality of spiral halftone dots, wherein saidspiral halftone dots comprise

(i) image pixels arranged as a first arc or as a plurality of arcs whichtogether represent a first spiral, and(ii) non-image pixels arranged as a second arc or as a plurality of arcswhich together represent a second spiral.

Compared to the concentric rings of US2007/0002384, the spiral dotscontain a longer, preferably open-ended ink channel, defined by thesecond arc or second spiral, which enables a better spreading of theink.

EP3461116 discloses a digital halftone screening method wherein aconventional halftone dot is split into separate clusters by a dotfunction and pixels are turned on or off within the clusters by acluster function. This method involves complex mathematics and cantherefore not be implemented in the wide installed base of prepresssystems having a limited data memory and/or processing power.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofreproducing a continuous-tone image with a printing press by means of ahalftone raster image having similar advantages as the raster imagecomprising spiral dots which is disclosed in copending applicationPCT/EP2018/079011, in particular a method which allows to save ink inthe lithographic or flexographic printing process and which can beimplemented in the wide installed base of prepress computer systems witha limited amount of memory and/or processing power.

This problem is solved by the method defined in claim 1, wherein asingle threshold array is used for the rendering of a continuous-toneimage, i.e. transforming a continuous-tone image to a halftone rasterimage which comprises regularly tiled halftone cells consisting of agrid of image pixels and non-image pixels. When the raster image isexposed on the coating of a printing plate precursor, the image pixelscorrespond to the printing (ink-accepting) areas of the plate, while thenon-image pixels correspond to the non-printing areas of the plate. Inat least a portion of the halftone cells, the image pixels are arrangedas multiple image clusters, which are defined herein as mutuallyseparated groups of more than 4 adjacent image pixels. The imageclusters allow to obtain a printed image of the same image density withless ink than with conventional screens wherein the image pixels aregrouped into a single cluster.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows 6 halftone cells, each consisting of a 16×16 grid ofpixels, in the upper half of the figure. These halftone cells were eachgenerated by the same threshold array in accordance with the method ofthe present invention. Said threshold array is the matrix shown in thelower half of the figure. Each halftone cell represents another relativeimage density, as indicated by the dot percentage shown below eachhalftone cell. The image pixels are represented by the black areas inthe figures, while the non-image pixels are represented by the whiteareas.

FIGS. 2 to 11 differ from FIG. 1 only therein that another thresholdarray is used in each figure.

FIG. 12 shows 6 tiled halftone cells, each comprising 4 groups of 3×3image pixels.

FIG. 13 and FIG. 14 show 6 tiled halftone cells which are identical tothe 70% halftone cells in FIG. 3 and FIG. 4 respectively.

FIG. 15 shows a halftone cell which comprises 3 image clusters,designated by the dashed lines A-C; the image pixels designated d-h donot form a group of more than 4 adjacent pixels and therefore do notcomply with the definition of an image cluster in accordance with thepresent invention.

FIG. 16 shows 6 halftone cells, each consisting of a 16×16 grid ofpixels, in the upper half of the figure and the threshold array whichwas used to generate these cells in the lower half. The threshold valuesin the array range from 1 to 16, contrary to the values in FIGS. 1-11which range from 1 to 256.

DESCRIPTION OF PREFERRED EMBODIMENTS Raster Image

The raster image produced in the method of the present invention issuitable for rendering a continuous-tone image (CT), i.e. it creates theillusion of a continuous-tone image (CT) on a printed copy. Thisrequirement implies that the screen frequency, i.e. the number ofhalftone cells arranged next to each other per length unit in thedirection that yields the greatest value, is above 40 lines per inch(LPI; 15.7 lines/cm), more preferably above 60 LPI (23.6 lines/cm) andmost preferably above 100 LPI (39.4 lines/cm). If the screen frequencyis below 40 LPI, the halftone dots become visible at viewing distance,also called reading distance, which is about 20 cm. Such low screenfrequencies are typically used in artistic screening, which is used fordecorative purposes such as patterned illustrations, wherein it isintended that the individual dots are visible to the naked eye. Rasterimages wherein the halftone dots are clearly visible at viewing distanceare therefore not suitable in the method of the present invention.

The above mentioned screen frequency defines the spatial frequency ofthe halftone cells comprised in the raster image. As explained above, ahalftone cell on its turn consists of a grid of pixels and the spatialfrequency thereof, called resolution, is expressed as dots per inch(DPI) or pixels per inch (PPI). In case the raster image is written bymeans of a scanning laser on e.g. a film or a printing plate, the pixelsare also called laser dots and the resolution then refers to the numberof laser lines per inch. The raster image produced in the method of thepresent invention has preferably a resolution larger than 600 DPI, morepreferably larger than 1200 DPI. Higher resolutions up to 9600 DPI mayalso be used, e.g. for the printing of security features.

Since a printing plate can transfer only a single colour, it is evidentthat the halftone raster image used in the method of the presentinvention is a monochrome image, which may represent a colour selectionof a multi-colour printing process, e.g. one of the 4 basic colours inCMYK printing.

The quality of the printed image is preferably checked with colourdensity measurement. The colour density values can then be used as inputparameters of the prepress computer system which produces the rasterimage, which is thereby adjusted so that the quality of the printedimage is improved in a subsequent press run and/or so that more ink issaved in a subsequent press run.

Halftone Cells

The raster image produced in the method of the present inventioncomprises regularly tiled halftone cells. The cells may be tiled along atriangular, rectangular or hexagonal grid and more preferably along asquare grid. FIGS. 13 and 14 each show examples of the preferredembodiment wherein 6 halftone cells are tiled as a square grid.

The halftone cells themselves also consist of a grid, more particularlya grid of pixels, which may be image pixels or non-image pixels. Thesepixels preferably have the shape of a regular polygon or convex polygon,e.g. a triangle, a square, a rectangle, a rhombus or a hexagon. TheFigures show examples of preferred embodiments, wherein the halftonecells consist of a grid of square pixels.

The raster image produced in the method of the present inventioncomprises halftone cells which have 2 or more image clusters, i.e.mutually separated groups of more than 4 adjacent image pixels. In amore preferred embodiment, the raster image comprises halftone cellshaving more than 2 image clusters, e.g. at least 3 or 4 image clusters,more preferably at least 5 and most preferably at least 6 imageclusters.

In the above definition of ‘image cluster’, the image pixels areconsidered to be adjacent if they share at least one edge of thepolygon. FIG. 15 shows three such image clusters: cluster A consists of7 adjacent image pixels; cluster B consists of 6 adjacent image pixels;and cluster C consists of 5 adjacent image pixels. The image pixels d, eand f are in contact with another image pixel but only by a corner ofthe square; as these pixels are not sharing an edge, they are notconsidered to be adjacent, as defined above. Groups of 4 or less imagepixels like group h neither constitute an image cluster in accordancewith the above definition.

FIG. 12 illustrates a further refinement of the definition of an imagecluster used herein. When halftone cells are tiled, groups of imagepixels in one halftone cell may connect with one or more adjacent edgesto another group of image pixels in another halftone cell. FIG. 12 showsan example wherein the group of image pixels designated 3 a connects tothe group designated 3 b. In accordance with the present invention, suchgroups shall not be regarded as separate clusters but together representa single cluster. As a result, the halftone cells represented in FIG. 12each contain only 3 image clusters.

Contrary to conventional AM dots which represent the same dotpercentage, the image clusters used in the present invention allow toobtain a printed image of the same image density with less ink. Thereasons for this advantage are not completely understood but theinventor of the present invention have systematically measured thatpress runs according to the method of the present invention consumesignificantly less ink compared to press runs with plates exposed with aconventional AM screen of the same original image. When compared with FMscreens, it is observed that a higher run length can be obtained,because the clusters are larger than the FM microdots and therefore lesssusceptible to wear on the press. FM screens consist either of aplurality of single image pixels or a group of four (2×2) image pixels,which degrade more easily than the image clusters used in the presentinvention.

Without being limited by the underlying mechanism, it is at presentassumed that the halftone cells having clustered image pixels asdescribed above, when compared to a conventional AM halftone dotrepresenting the same relative image density, either absorb a thinnerink film on the ink accepting areas of the plate and/or provide a betterspreading of the ink film into the empty (non-image) areas between theimage clusters. The ink saving effect has been observed with variousimages and various plates. Ink savings of about 10% were frequentlyobtained. Ink and paper are the major cost factors of a printer, so evena reduction of the ink consumption by a few percent represents a highcost saving. Optimization of the raster image in relation to thesubstrate that is to be printed, e.g. by adjusting the number, size,shape and/or distribution of image clusters in the halftone cells, canlead to even more ink saving, in the range from 10 to 20% compared toconventional AM screening.

Less consumption of ink provides additional advantages resultingtherefrom, e.g. faster drying and/or less energy consumption by dryingequipment such as curing units and ovens. Faster drying is particularlybeneficial for printing on uncoated plastic foils and in newspaperprinting. The better spreading of the ink also reduces ink setoff, i.e.the transfer of ink from one printed copy to the back side of anothercopy lying on top of it, for example in the press delivery tray.Shine-through, also called print-through, whereby images become visibleat the backside of the substrate, e.g. a thin, ink absorbing substrateas used for the printing of newspapers, is reduced as well. For allthese reasons, the method of the present invention is especiallyadvantageous when performed on a perfecting press, i.e. a press thatallows the simultaneous printing on both sides of the substrate in onepass through the press.

In a preferred embodiment, the image clusters are not distributedrandomly over the halftone cell, but are concentrated locally, so thatthe image clusters together mimick a conventional AM dot and the methodmaintains the advantages of AM screens as much as possible. The imageclusters may be concentrated in e.g. a quarter section of the halftonecell. As a result, that quarter section represents a higher relativeimage density than the other sections of the halftone cell. Morepreferably, one quarter section of the halftone cell has a relativeimage density which is at least twice the relative image densityrepresented by the halftone cell as a whole. FIG. 5 shows an example ofsuch an arrangement: the 8×8 pixels around the centre of the cell definea quarter section, indicated by thick lines, which has a much higherdensity of image pixels than the cell as a whole. The higherconcentration of image pixels should not necessarily be positioned nearthe centre of the cell: FIG. 3 and FIG. 4 show different localconcentrations of image pixels, which however represent the samehalftone image when these cells are tiled next to each other, as shownin FIGS. 13 and 14 respectively.

The halftone raster image used in the present invention may contain acombination of different types of halftone cells, e.g. halftone cellswith multiple image clusters, as defined above, combined withconventional halftone cells, e.g. AM halftone cells wherein all imagepixels are grouped in a single cluster. One or more parts of the imagemay also be represented by FM screens. In a highly preferred embodimentof the present invention, the highlights and midtones of the image, i.e.the subset of all the halftone cells in the image which represent arelative image density of less than 50%, consist entirely of halftonecells having 2 or more image clusters as defined above. In anotherembodiment, only a portion of the halftone cells which representhighlights and midtones of the image contain 2 or more image clusters.Said portion may be as low as 5%. Preferably, said portion is at least10%, more preferably at least 25% and even more preferably at least 50%.

In another preferred embodiment of the present invention, the imageclusters are arranged along a path which represents a loop or a spiral.The loop may be a square, a rectangle, a ring or an ellipse. FIG. 10represents halftone cells wherein the clusters are arranged along thepath of a ring, while FIGS. 6-9 represent halftone cells wherein theclusters are arranged along the path of a spiral, which are referred tohereafter as “spiral dots”. The image pixels and clusters represent afirst spiral of ink-accepting areas and thereby also define a secondspiral of non-image pixels represented by the empty space between thewindings of the first spiral. Without being bound by theory, it isassumed that the ink, which is transferred from the spiral dot onto asubstrate such as a paper sheet, can flow into the second spiral, whichseems to act as an ink-accepting channel. A spiral dot has the uniquecharacteristic that all its non-image pixels are in contact with oneanother so that a higher extent of ink spreading is obtained than withthe concentric rings of US2007/0002384. In the midtones and highlightsof the image, the second spiral is preferably open ended, so that it canact as an ink channel that guides the ink out of the spiral dot: whenthe ink is pressed onto the substrate, part of the ink may be expelledfrom the second spiral and becomes visible upon magnification of printedimages as the release of a small quantity of ink at the outlet of thechannel. The open channel enables a further spreading of the ink and, asa result, more ink saving and still faster drying. A similar effect canbe obtained by the halftone cells represented in FIG. 10, wherein thering has gaps between the clusters which allow to evacuate ink.

It shall be clear to the skilled person that the same dot coverage canbe produced with different spiral dots of the same overall size: a dotconsisting of just one winding of a first spiral of a certain thicknessproduces the same coverage as a dot with more windings of a first spiralof a lower thickness. The thickness of the first spiral may also varywithin the same dot, e.g. smaller at the centre than at the edge of thedot. The winding of the spirals may be clockwise or counter clockwise,and both these embodiments can be combined in the same raster image. Thestart angle of first spiral, at the centre of the dot, is preferably thesame for all spiral dots in the image or can be chosen randomly by arandom number generator. More details of suitable spiral dots aredisclosed in copending application no. PCT/EP2018/079011, filed on 23Oct. 2018.

Threshold Array

According to the present invention, the raster image is generated by asingle threshold array. Digital halftoning by means of a thresholdarray, sometimes also called threshold matrix or threshold tile, isknown in the art. When used for multi-colour printing, the number ofthreshold arrays is preferably the same as the number of colour channelsin the original continuous-tone image. Digital halftoning with athreshold array typically implies that the original image is sampledinto cells which are mapped on the threshold array. The local densityvalue of the original image is then compared with each of the values inthe array (if necessary, the density range of the original image isadjusted so that it is equal to the range in the array). The outputpixel is set to 0 if the original density value is lower than thethreshold value of said pixel. Otherwise, if the density value is equalto or exceeds the threshold value, the output pixel is set to 1. Thesesteps are repeated for all cells in the image.

The dimension of the array, i.e. the number of pixels per halftone cell,may depend on various factors such as the resolution of the image setterand the desired quality of the printed image. The array is preferablyarranged as a square (n×n dimension) or a rectangle (m×n, with m>n).FIGS. 1-6 show examples of square threshold arrays having a dimension of16×16 locations, wherein each location contains a threshold value in acertain range. In these particular examples, the threshold value range(1-256) is equal to the number of locations (16×16) in the array; inother embodiments, such as the example shown in FIG. 16, the value rangemay be lower than the number of locations and then each individual valuemay occur at multiple locations of the array.

In order to produce higher relative image densities, the threshold arrayis designed in such a way that the number and/or the size of the imageclusters increases in line with the corresponding density of theoriginal image. For example, FIG. 1 shows a cell which represents arelative image density of 25% with 7 image clusters, of which the sizegrows at increasing image densities. FIG. 3 shows a 10% cell with 5image clusters, wherein both the size and the number of clusters grow athigher image densities. While the image density of the halftone cellgrows by adding image pixels, It is preferred to keep the non-imagepixels together in non-image clusters, which are defined as mutuallyseparated groups of adjacent non-image pixels. By keeping the number ofnon-image clusters as low as possible in the shadows of the image, theink can spread to a higher extent than when the non-image pixels areisolated or distributed over multiple clusters. In that way, ink canalso be saved in the shadow areas of the image.

In the preferred embodiment using spiral dots, the relative imagedensity can be increased by growing the length and/or the thickness ofthe first spiral (which comprises the image pixels), as shown in FIG. 6,or by inserting image pixels inside the second spiral; or by acombination of any of these methods. In the shadows of the image, moreimage pixels are added to the halftone cell in such a way that the inkchannel formed by the second spiral (which comprises the non-imagepixels) shrinks and/or becomes thinner, as shown in FIGS. 6 and 9.

Printing Plate

The printing plates used in the method of the present invention areobtained by the exposure of the halftone raster image on a light- orheat-sensitive material called printing plate precursor. The plateprecursor can be positive or negative working. A positive plateprecursor has a coating which after exposure and development accepts inkat non-exposed areas and no ink at exposed areas. A negative plateaccepts ink at exposed areas and no ink at non-exposed areas. The imagepixels shown in the Figures define ink-accepting areas of the plate andthus correspond to exposed or non-exposed areas of a negative orpositive plate precursor respectively.

The plates used in the method of the present invention are preferablylithographic or flexographic printing plates. Lithographic plates aretypically obtained by exposing the halftone raster image on the light-or heat-sensitive coating of a printing plate precursor by means of ascanning laser, preferably a violet or a near infrared laser, or anotherdigitally modulated light source, such as a digital mirror device, LCDor LED display. After processing the exposed precursor with a suitabledevelopment liquid, a lithographic printing plate carrying the rasterimage of the present invention is obtained. That plate can then bemounted on a lithographic printing press, preferably an offset press,wherein ink is supplied to the plate which is then transferred onto thesubstrate to be printed. Alternatively, the exposed precursor can bemounted directly on the press, i.e. without any preceding liquidtreatment or other development, and the development of the image maythen occur by means of the ink and/or fountain which is supplied to theplate at the start of the press.

Flexographic plates are generally obtained by UV exposure of aphotopolymer coating, typically with a UV lamp through a mask which canbe a graphic film in contact with the photopolymer coating or an in-situmask that is present on top of the photopolymer coating. The mask ispreferably obtained by exposing the halftone raster image on the film oron the in-situ mask layer by means of a scanning laser, preferably anear infrared layer.

The substrate on which the raster image may be printed can be of anykind, e.g. plastic films or foils, release liner, textiles, metal,glass, leather, hide, cotton and of course a variety of paper substrates(lightweight, heavyweight, coated, uncoated, paperboard, cardboard,etc.). The substrate may be a rigid work piece or a flexible sheet, rollor sleeve. Preferred flexible materials include e.g. paper, transparencyfoils, adhesive PVC sheets, etc., which may have a thickness less than100 micrometres and preferably less than 50 micrometres. Preferred rigidsubstrates include e.g. hard board, PVC, carton, wood or ink-receivers,which may have a thickness up to 2 centimetres and more preferably up to5 centimetres. The substrate may also be a flexible web material (e.g.paper, adhesive vinyl, fabrics, PVC, textile). A receiving layer, forexample an ink-receiving layer, may be applied on the substrate for agood adhesion of the reproduced image on the substrate.

1-10. (canceled)
 11. A method of reproducing a continuous-tone imagewith a printing press, the method comprising the steps of: (i) digitalhalftoning by transforming the continuous-tone image into a halftoneraster image comprising regularly tiled halftone cells which consist ofa grid of image pixels and non-image pixels; (ii) exposing the halftoneraster image on a printing plate precursor; (iii) developing theprinting plate precursor into a printing plate; (iv) mounting theprinting plate on a printing press; (v) supplying ink to the printingplate and transferring the ink from the printing plate to a substrate;wherein step (i) is carried out by sampling the continuous-tone imageinto cells which are mapped onto a single threshold array and at least aportion of all the halftone cells in the raster image comprise 2 or moreimage clusters, image clusters being defined as mutually separatedgroups of more than 4 adjacent image pixels.
 12. The method of claim 11,wherein step (iii) is carried out after step (iv).
 13. The method ofclaim 11, wherein the portion comprises at least about 10% of all thehalftone cells having a relative image density of less than about 50%.14. The method of claim 11, wherein the portion comprises at least about50% of all the halftone cells having a relative image density of lessthan about 50%.
 15. The method of claim 11, wherein the portion consistsessentially of all the halftone cells having a relative image density ofless than about 50%.
 16. The method of claim 11, wherein the halftonecells of the portion comprise at least 4 image clusters.
 17. The methodof claim 13, wherein the halftone cells of said portion comprise atleast 4 image clusters.
 18. The method of claim 16, wherein the imageclusters in the halftone cells of the portion are arranged along a pathwhich represents a loop or a spiral.
 19. The method of claim 16, whereinthe halftone cells of the portion comprise quarter section whichrepresents a relative image density which is at least twice the relativeimage density represented by the halftone cell as a whole.
 20. Themethod of claim 11, wherein the printing plate is a lithographic or aflexographic printing plate.
 21. The method of claim 12, wherein theprinting plate is a lithographic or a flexographic printing plate. 22.The method of claim 16, wherein the printing plate is a lithographic ora flexographic printing plate.
 23. The method of claim 16, furthercomprising a step (vi) wherein colour density values are measured on thesubstrate and the colour density values are used in step (i) to adjustthe halftone raster image.
 24. The method of claim 20, furthercomprising a step (vi) wherein colour density values are measured on thesubstrate and wherein the colour density values are used in step (i) toadjust the halftone raster image.
 25. The method of claim 23, whereinthe image clusters are adjusted in step (vi) by changing their number,size, shape and/or distribution in the halftone cells.
 26. The method ofclaim 24, wherein the image clusters are adjusted in step (vi) bychanging their number, size, shape and/or distribution in the halftonecells.